The present invention relates to a device for welding two films, with a heated sealing or welding arrangement cooperating with an opposed tool, the films being arranged between the sealing or welding arrangement and the opposed tool and the sealing or welding arrangement and the opposed tool being movable relative to each other.
Devices of the above-mentioned type are known. The sealing or welding arrangement is usually moved upwardly from below while the opposed tool is moved downwardly from above and, in particular, is integrated into the arrangements which are required for forming loops.
It is pointed out that it is of lesser importance for the device just described, and for the invention, whether the arrangement which joins the ends of the films is formed as a sealing arrangement or as a welding arrangement, so that in this context the one procedure or configuration is entirely interchangeable with the other procedure or configuration.
It is also inessential to the application of the invention whether the device is used for welding two films or two film ends, or both ends of one film, and so on. It is also possible, using the device, to weld together both ends of two areas of the same film, for example, to form a hose or similar part, in order to benefit from the effect according to the invention, and this can apply, for example, to pieces of film as understood in accordance with the invention. It is also possible according to the invention to weld together more than two films or pieces of films.
In the known devices the sealing or welding device comprises a heating blade the upper edge of which has a wedge-like configuration. This heating blade has the function of severing the films as it makes contact with them and at the same time of welding together the film ends produced by the severing action. In this process the film ends forming the loop are firstly to be welded together, so that an endless form is produced. Secondly, the film ends fed from above and below are to be re-connected after separation of the loop, so that a curtain is produced and the work cycle can be repeated. Such devices as those described at the outset are known, for example, in bandolier machines.
For welding of PE films, temperatures of approximately 250xc2x0 C. are usually necessary. The heating blade or heating mirror is either permanently heated or intermittent heating of these arrangements for short periods is provided, in which case the relatively large mass of the heating blade entails a corresponding thermal inertia. For this reason the sealing process requires a certain time.
In known devices, an interval of about 0.5 sec. is required for sealing, half of this time being needed for heating and the other half for cooling, and in particular for bonding the film ends.
For example, if a bandolier machine of the type in question executes 35 work cycles per minute, a time of 1.71 sec. is available for one work cycle. The time interval required for the complete welding process therefore accounts for about one-third of the total time.
Furthermore, the heating blade used in such machines is a source of danger or injury, as it is usually heated to a temperature of more than 200xc2x0 C.
In the known machine the heating blade, as already described, is moved upwardly from below during the work process. In the upper part of the movement the blade acts on the films. The not inconsiderable mass of the heating blade and the associated arrangements requires a sufficiently powerful drive system to which the corresponding machine parts must be adapted. In addition, means for aligning or fixing the films one above the other are necessary in the known machines. For this purpose suitable tensioning elements or clamping devices are used, the operation of which must be incorporated in the periodically-occurring welding procedure, which operation, of course, takes time. Furthermore, the use of these additional elements is correspondingly expensive and also requires additional space, which is difficult to provide, especially because the welded seam, for example, on a ready-packed product, must be arranged as close as possible to the product.
It is an object of the present invention to provide an improved device for welding films/pieces of film of the type described above in which the above disadvantages are reduced or substantially obviated. It is a further object to provide a device in which that a higher cycle frequency is achieved.
The present invention provides a device for welding two films or pieces of film, with a heated sealing or welding arrangement cooperating with an opposed tool, the films being arranged between said sealing or welding arrangement and said opposed tool and said sealing or welding arrangement and said opposed tool being movable relative to each other, wherein said opposed tool has elastic material on its front face facing towards said film and/or said front face of said opposed tool facing towards said film is of elastic configuration, and in that said elastic material and/or said front face yields elastically at least partially when said sealing or welding arrangement and said opposed tool are moved together.
In the device according to the invention, when said opposed tool necessary to the welding process is advanced, said films are also fixed in place, one upon the other, by the same movement, either simultaneously or before or afterwards. By combining two operations of fixing the films and advancing said opposed tool for the welding process, time is saved in the periodic welding process and it is possible, in principle, to operate the device at higher cycle frequencies.
According to the invention the device is used for welding, that is, for joining two pieces of film without having to destroy said film. In a preferred embodiment of the invention combined cutting and welding, that is, a process in which the film is severed, is also possible.
In order to join the films, the opposed tool and the sealing or welding arrangement are movable relative to each other. Through the elastic configuration of the face of the opposed tool facing the film, it is ensured that, regardless of the exact thickness of the films or the relative position of the sealing or welding arrangement in relation to the opposed tool, sufficient pressure or force is always exerted on the film so that the desired function, for example, welding or combined cutting and welding, is achieved. In the prior art, complex adjustment of the precise final positions of the moving parts in relation to each other was necessary in order to avoid, in particular, shearing of the films by the moving parts without cutting and welding being achieved. Conversely, adjustment was also necessary to ensure that no gap was left between the opposed tool and the sealing arrangement, which gap would make it impossible to join the films. The design advantage as described above also makes possible a further advantage, in that the complex adjustment processes otherwise needed, especially when changing films or carrying out maintenance work, can be omitted.
In a preferred embodiment of the device according to the invention said opposed tool includes means by which the degree of elasticity of said opposed tool and/or of said front face is varied and, in particular, is reduced, so that this zone exerts increased pressure on said film when said sealing or welding arrangement and said opposed tool are brought together. The bringing together of said sealing or welding arrangement and said opposed tool causes said material to yield towards said opposed tool, because of its elasticity. As a result of this springing effect, an essentially predetermined force is produced between said opposed tool and said sealing or welding arrangement. In the further development according to the invention this force is varied locally in order to achieve special effects, for example, by applying increased pressure to certain portions in order to provide better thermal contact or to exert corresponding shear forces on said film in order, for example, to sever said film more easily or quickly. However, increased elasticity can also result, and reduced pressure can be provided, for example, in order to prevent contact between or sealing of said films despite said heated tool face. The effect described above is used in particular, however, to provide zones which not only weld but serve for cutting and welding. As means for changing the elasticity of said elastic material, inserts, beads, additional layers or weakened areas of said elastic material may be utilised.
In a preferred embodiment of a device according to the invention said opposed tool is formed as a rail and the means for varying the elasticity of said elastic material are arranged along said rail. The rail-like opposed tool is aligned, in the feed direction, transversely to the film strip and, if appropriate, is movable at right angles to it. The sealing or welding arrangement cooperating with the opposed tool is correspondingly positioned and, if appropriate, is also movable or stationary in relation to it.
In the invention the welding and/or severing of the film ends is essentially achieved by an appropriate configuration of said opposed tool. In particular, said sealing or welding arrangement is arranged without a drive mechanism, only said opposed tool being advanced towards said sealing or welding arrangement.
Considerable advantages are obtained through the invention. In addition to the fact that said sealing or welding arrangement is no longer moved but is fixed in place, so that its drive system is omitted, owing to the fact that said rail consists of elastic material, the different necessary work processes can logically be carried out successively or simultaneously, or in a manner whereby the different work processes overlap in time, without impairing the final result. For example, combined cutting and welding can take place when said films are just being clamped. As a result, the process cycle can be accelerated.
The silicone rubber used for example in the device according to the invention as said elastic material, can function up to the required temperatures of approximately 300xc2x0 C. and also has sufficient durability. In particular, it is sufficiently elastic, so that the result sought is achieved merely by moving said opposed tool against said sealing or welding arrangement.
The invention preferably makes use of a sealing or welding arrangement which is so configured that it is brought to the required welding or cutting temperature in a short time but then cools sufficiently quickly for said sealing or weld seams to be stable. Said heating rail itself is located at the centre of said sealing or welding arrangement and is wide enough to cooperate with said cutting and welding portions of said opposed tool.
In a preferred embodiment of the device according to the invention, said heating rail is surrounded by several layers of ceramic and/or plastic material, so that a relatively small thermal mass is obtained. This sharply reduces the thermal inertia of the system.
In the arrangement according to the invention an interval of only up to 0.4 sec. is required for heating. A current pulse of 0.05 to 0.15 sec. is preferably used to heat said heating rail to working temperature. Said heating rail cools in approximately the same interval. The sharp reduction in the time required for sealing allows the machine to achieve a substantially higher number of cycles per minute. To realise these fast heating and cooling times, special configurations of the sealing or welding unit are proposed according to the invention.
In a further embodiment of the device according to the invention the contours of the operating faces of said opposed tool and of said sealing or welding arrangement are so adapted to each other that the zone for clamping projects beyond the others so that this zone becomes effective before the others.
In this preferred embodiment of the device according to the invention said opposed tool presses said two superimposed films against said sealing or welding arrangement. Because of the projection of the clamping zones, clamping and thus fixing of said films is achieved. In particular, at the moment of fixing said film by said clamping zones, there is practically no contact between said cutting zone and said heating arrangement. But as said opposed tool is in motion, contact between said cutting zone of said opposed tool and said two superimposed films, which are thereby pressed against said heating arrangement, occurs immediately after the clamping process.
For this purpose said rail of elastic silicone rubber or similar material has a projecting rib at right angles to the feed direction of said films, which rib presses said films against said heated sealing or welding arrangement. This rib acts as a means for changing the elasticity of said elastic element. It is not necessary for said sealing or welding arrangement to be wedge-shaped, as it was described at the outset with reference to the prior art. Said sealing or welding arrangement can, for example, be flat in this area.
Said projecting rib on said silicone rubber rail is preferably wedge-shaped in cross-section, so that the welding and cutting of said films occurs in a relatively small zone. The required heat which must act on said films thus remains low.
According to a further feature of the device according to the invention substantially flat faces are arranged on either side of said rib and form the zone for welding or sealing. In the feed process, therefore, after the severing of said two films, said films are pressed against said heating rail. The two processes occur close together in time, overlapping or simultaneously, as determined by the feed rate of said opposed tool and the elasticity of said silicone rubber rail.
The short heating phase of said heating rail already passes into a cooling phase at this stage of the process, as the temperature for sealing is somewhat lower than the temperature for cutting. It is favourable in this regard that said heating rail has low thermal inertia, that is, said heating conductor is relatively narrow and of low mass so that not too much material is heated. It is also favourable it a cooling arrangement, in particular a water cooling system, by which the reduction of temperature after sealing can be further accelerated, is provided for said heating rail.
The welding and sealing zones on said silicone rubber rail running along either side of said cutting projection are preferably substantially flat.
Outside the zones for welding and sealing projecting beads are preferably provided to clamp said films during the interaction with said sealing or welding arrangement.
Said sealing or welding arrangement preferably has recesses cooperating with said clamping zones on said opposed tool. In particular, these recesses which cooperate with said beads are only partly matched to these beads. This causes deformation of said beads and an improved clamping effect. Obstructions by very small foreign bodies cannot have a detrimental effect.
Alternatively to providing said opposed tool with means for altering the elasticity of said elastic material, for example, a rib or a wedge, it is also possible to give said face of said opposed tool a flat or spherical form, for example, as part of a cylinder jacket face or suchlike configuration. In particular, the smooth, flat form also produces the effect, in accordance with the invention, that, for example, welding or combined cutting and welding is achieved by appropriate triggering of said heating rail. However, the use of a means of changing the elasticity, such as an insert, rib or similar means, as described later, increases the mechanical pressure in this zone and under some circumstances resulting in faster processing as well as helping to increase the cycle rate.
In the device according to the invention, said opposed tool comprises elastic material for example, for said rail. Elastomers, caoutchouc, rubber or silicone rubber or similar materials are preferably used for this purpose. By the term xe2x80x9cElastomersxe2x80x9d are meant polymers with rubber-elastic behaviour, which are repeatedly stretched at room temperature and after storage return to approximately their initial length. No particular elastomer is exclusively suitable for use in the device according to the invention; all known elastomers in this branch of technology, for example, rubber based on polyester urethane, ethylene acrylate, ethylene-propylene copolymers and so on, can be used. The favourable thermal properties of silicone rubber (e.g. FVMQxe2x80x94fluosilicone rubber) have already been mentioned. It is also possible, however, in addition to synthetically produced materials, to use natural products such as caoutchouc or rubber which, if necessary, are shielded with a heat-resistant material and produce elastic bedding. Plastics with different chemical compositions, which are sufficiently elastic and can withstand the temperatures applied, can also be used.
The portions of said rail used for clamping are so formed and arranged, in relation to said other portions used for welding or sealing, that they project by about 0.2 mm to 1.5 mm beyond said portions used for welding or sealing.
A similar relationship can be achieved if, in the zone of said heating arrangement, for example, a corresponding profile is provided to ensure that when said opposed tool is advanced the clamping process occurs sufficiently ahead of the moment of welding of sealing. Said rail used for cutting can be formed and arranged in an analogous way.
The zone of said rail used for cutting is so arranged in a preferred embodiment that it projects by about 0.4 mm to 1.3 mm beyond the zones for welding or sealing.
The invention can be varied in different ways; for example, it is not absolutely necessary that the surfaces of the zones on said rail also consist of silicone rubber. It is possible to form these zones entirely or partially from layers consisting of a different material, for example metal. These layers are mounted elastically and resiliently on said rail, for example, through being supported on springs or similar means.
It is recommended that said opposed tool is formed so that said rail made of elastic, silicone rubber or similar material is mounted interchangeably on a carrier. With such a configuration it is possible to exchange said elastic material simply, if it is worn or damaged.
The invention relates not only to the device as described above but extends likewise to include a bandolier machine in which a feed arrangement advances the merchandise to be wrapped against a film curtain which forms a loop around said merchandise, and in which a device as described above is used, and said heated sealing or welding arrangement which cooperates with an opposed tool severs said loop formed behind said merchandise from said film curtain and joins said film ends of said loop and of said curtain.
With such a machine the cutting or combined cutting and welding and the welding are carried out substantially simultaneously. For this it is preferred that said opposed tool be formed by a profiled rail with elastic silicone rubber as said elastic material, said front face of which, facing said film, is either substantially flat or has zones for clamping or for welding or severing.
An embodiment of a bandolier machine will now be described with reference to the accompanying drawings, in which:
FIGS. 1 to 4 are a schematic illustration of the essential parts of a bandolier machine according to the invention in different positions during a work cycle;
FIG. 5 shows an enlarged view of a detail of the device according to the invention and
FIGS. 6a to 6i show side views of a detail of the opposed tool of the device according to the invention.